1. Field of the Invention
Various embodiments of the present invention generally relate to a keyboard, video, mouse (KVM) switch that controls computers with an input-output unit having a keyboard, a mouse and a display.
2. Description of the Related Art
A KVM switch is connected between an input-output unit and computers. The KVM switch makes one of the connections between the input-output unit and the computers an active. A user selects and uses one of the computers that is to be connected by using the KVM switch.
FIG. 1 illustrates a structure of a conventional KVM switch. The KVM switch has server units 2a through 2n (“n” is a given value), user units 4a through 4n (“n” is a given value), and a main unit 3. Hereinafter, a given one of the server units is referred to as a server unit 2. A given one of the user units is referred to as a user unit 4.
The server unit 2 is provided on the side of computer units 1a through 1n (“n” is a given value). Hereinafter, a given one of the computers is referred to as a computer unit 1. The user unit 4 is provided on the side of input-output units such as monitors 5a through 5n (“n” is a given value), keyboards 6a through 6n (“n” is a given value), and mouses 7a through 7n (“n” is a given value). Hereinafter, a given one of the monitors is referred to as a monitor 5. A given one of the keyboards is referred to as a keyboard 6. A given one of the mouses is referred to as a mouse 7. The main unit 3 is provided between the units, and has a switch that switches connections between the computer unit 1 and the input-output unit.
A picture signal is output from the computer unit 1, and is fed into the main unit 3 via the server unit 2. In the main unit 3, a matrix switch 12 selects the user unit 4 to be connected. The picture signal is fed into the monitor 5 via the user unit 4.
There is a case where a cable length between the main unit 3 and the user unit 4 is a few hundreds meters. In this case, there is occurred a high degradation and a delay in the picture signal fed into the user unit. Therefore, there is provided a picture signal controller that controls a picture signal in the user unit.
FIG. 2 illustrates a structure of the main unit 3. The main unit 3 has picture signal input portions 11a through 11n (“n” is a given value), the matrix switch 12, switches 13a through 13n (“n” is a given value), picture signal output portions 14a through 14n (“n” is a given value), a microcomputer 15, a reference signal generator 16 and a keyboard/mouse signal generator 17.
Picture signals from the server units 2a through 2n are fed into the picture signal input portion 11a through 11n. The matrix switch 12 redirects the output of the picture signal. The switches 13a through 13b output one of the picture signal from the matrix switch 12 and a reference signal from the reference signal generator 16. The picture signal output portions 14a through 14n output the picture signals from the switches 13a through 13n or the reference signal to the user unit 4. The microcomputer 15 controls the switches 13a through 13n and the keyboard/mouse signal generator 17. The reference signal generator 16 generates the reference signal. The keyboard/mouse signal generator 17 generates a control signal of the keyboard/mouse.
FIG. 2 shows the picture signal input portions 11a and 11b of the picture signal input portions 11a through 11n, shows the switches 13a and 13b of the switches 13a through 13n, and shows the picture signal output portions 14a and 14b of the picture signal output portions 14a through 14n.
The microcomputer 15 controls the switches 13a and 13b so that the reference signal from the reference signal generator 16 is fed into the picture signal output portions 14a and 14b, at a controlling timing of the picture signal.
FIG. 3 illustrates a structure of the user unit 4a of the user units 4. The user unit 4a has a picture signal input portion 21a, a picture signal controller 22a, a picture signal output portion 23a, an A/D converter 24a and a microcomputer 25a. 
When a reference signal is output from the main unit 3, the user unit 4a receives the reference signal with the picture signal input portion 21a and outputs the reference signal to the picture signal controller 22a. The picture signal controller 22a outputs the reference signal to the picture signal output portion 23a. 
The A/D converter 24a A/D converts the reference signal from the picture signal controller 22a, and outputs the reference signal to the microcomputer 25a. The microcomputer 25a controls the picture signal controller 22a so that an output signal from the picture signal controller 22a is optimized based on an output level of the reference signal.
Japanese Patent Application Publication No. 2005-278085 (hereinafter referred to as Document 1) discloses a system that has a reference signal generator in a transmitter and controls a picture quality automatically in a receiver.
Japanese Patent Application Publication No. 9-219803 (hereinafter referred to as Document 2) discloses a cable compensator that has a reference signal generator in a transmitter and controls a gain automatically in a receiver.
A distance between the server unit and the main unit is longer or shorter as well as a distance between the main unit and a user unit, in a case where the KVM switch is structured with the server unit, the main unit and the user unit.
The KVM switch controls a gain of the picture signal controller 22a, in view of a given distance between each of the units. A signal voltage level is not reduced to a required level and a halation may be occurred if the distance between the server unit and the user unit is extremely short, even if the gain of the picture signal controller 22a is controlled. In contrast, there is a problem that a desirable signal voltage level may not be obtained in a case where the signal voltage level is controlled between the main unit and the user unit in a conventional method, if the distance between the server unit and the main unit is longer. And the arts disclosed in Document 1 and Document 2 do not solve the problem.